According to the Animal Rescue League (ARL) of Boston, 47 grackles - a type of songbird - crashed to the ground earlier this month; 32 died, either at the scene or as rescuers tried to save them, while the remaining 15 birds are in "good condition".

Even more perplexingly, the deaths of two neighbourhood cats have also been linked to the incident.

"When I arrived, birds would fly, like from a house to a tree, they would flop in the tree and they would fall to the ground," ARL spokesman Alan Borgal explained. "The weaker ones were just falling right out of the sky."

The dead grackles have since been sent to Tufts University to establish whether the cause of death was due to a virus, environmental pollution or intentional poisoning.

"We don't know what is going on," explained John Meaney, from Boston's Inspectional Services office. "So we are investigating all avenues."

In the meantime, residents have been advised to keep their cats and dogs indoors, and to keep a close eye on what they may be eating.